spi: bcm2835: fix kbuild compile warnings/errors and a typo

fixes several warnings/error emmitted by the kbuild system:
* warn: cast from pointer to integer of different size
  using size_t instead of u32
* error: 'SZ_4K' undeclared
  moved to PAGE_SIZE and PAGE_MASK instead

Review showed also a typo in the same code where tx_buff
was checked twice instead of checking both rx and tx_buff.

Reported by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>